1. Field of the Invention
The present invention relates to a process for purifying pentachloronitrobenzene (sometimes referred to hereinafter as PCNB), by a distillation process which reduces the hexachlorobenzene (sometimes referred to hereinafter as HCB) content of the PCNB starting material to a concentration below 0.5%, and preferably to a concentration below the 0.1%. The present process also reduces the content of tetrachloronitrobenzenes (sometimes referred to hereinafter as TCNBs) and any lower chloronitrobenzenes that may have been present in the PCNB starting material to traces, thereby forming an essentially pure PCNB product.
2. Description of the Prior Art
Pentachloronitrobenzene is a commercially important fungicide for application to soil and seeds for controlling various plant diseases, specifically those caused by botrytis, fusarium, rhizoctonia and anthracnase.
Two industrial processes exist for the manufacture of PCNB. The first process, and by far the most important, involves the chlorination of nitrobenzene in chlorosulphonic acid using iodine as a catalyst. This process is described, for example, by Thurston in the "Fiat Formal Report No. 949." Lojewski, in the U.S. Pat. No. 3,026,358, disclosed a variation of this process, wherein chloronitrobenzene is used as the starting material in an attempt to reduce the final HCB content. The second commercial process for the production of PCNB is the nitration of pentachlorobenzene with nitric acid in sulfuric acid. This process is described by Breaux in U.S. Pat. No. 4,026,955, and by others, for example, U.S. Pat. Nos. 4,057,590; 4,138,438; and 4,147,732. The PCNB obtained with these prior art commercial processes, as described above, contains HCB in concentrations above 0.5%, by weight even with mild reaction conditions that severely affect productivity.
It has been demonstrated that HCB is an animal carcinogen. Consequently, the Office of Pesticides and Toxic Substances of the Environmental Protection Agency has set a deadline of Apr. 28, 1988 to industry manufacturers to implement new technologies to reduce the HCB level in PCNB to 0.1% by weight or less.
Recently, several processes for manufacturing PCNB having a reduced HCB content have been patented. Included among these processes are the conversion of HCB back to PCNB using pentachlorothiophenol (U.S. Pat. Nos. 4,454,362 and 4,461,918) and using pentachlorobenzonitrile (U.S. Pat. No. 3,984,487). While these processes do represent significant advances in producing relatively pure PCNB, they have the disadvantage of using HCB as a starting material, thereby requiring two additional steps. To date, a commercially acceptable process capable of successfully separating HCB from PCNB has not been described.